Method for producing air-permeable composite sheet

ABSTRACT

It is an objective of the present invention to provide a method for producing an air-permeable composite sheet which has excellent mechanical strength and compression resistance as well as PTFE&#39;s innate property such as air-permeability, water repellency, heat resistance and chemical resistance; and a filter and a fabrics material containing the air-permeable composite sheet produced by the method as a constituent material. The method for producing an air-permeable composite sheet according to the present invention is characterized in comprising steps of filling pores of a porous PTFE sheet with a hardenable material solution; hardening or semi-hardening the porous PTFE sheet filled with the hardenable material solution; and expanding the hardened or semi-hardened porous PTFE sheet.

FIELD OF THE INVENTION

The present invention relates to a method for producing an air-permeablecomposite sheet, and a filter and the like containing the air-permeablecomposite sheet.

BACKGROUND

A porous PTFE sheet exhibit a specific properties of passing gases suchas water vapor therethrough, while not permeating water, so that thesheet is excellent in moisture permeable waterproof property, by which alow humidity can be kept inside even in rain and in sports. Therefore, aporous PTFE sheet has been widely used as fabrics materials of shoes,wear and the like. In addition, a porous PTFE sheet has been widely usedalso as various filter materials, since a porous PTFE is excellent inheat resistance and chemical resistance, and the pore size thereof isadjustable.

On the other hand, a porous PTFE sheet is not sufficient in compressiveresistance, mechanical strength and abrasion resistance due to thestructural issue that a porous PTFE sheet is made of a fluorinated resinand is porous. Hence, technology for improving the strength of a porousPTFE sheet has been studied.

For example, Patent Documents 1 to 9 disclose a porous PTFE sheet ofwhich pores are filled with a filler, and the production methodsthereof.

However, the porous PTFE sheets described in the Patent Documents areproduded by blending a PTFE powder with a filler powder, andpreliminarily-molding the mixture, and then expanding it. As a result,the sheets have a structure in which filler powders are adsorbed on thepores of the porous PTFE sheet, whereby it is difficult to say that theinherent defects of porous PTFE sheet are sufficiently improved.

Additionally, a sheet made of ultra-high molecular weight polyethylenehas been known as a porous sheet with wear resistance. However, it isdifficult to make the sheet thin, since the sheet is produced by cuttinga sintered body. In addition, the sheet is made of polyethylene and thuslacks in compressive resistance and heat resistance.

Patent Document 10 discloses a composite material produced bycompressing a thermoplastic resin fiber with heating to produce a porousmaterial, impregnating the porous material with a solution of anotherthermoplastic resin, and then cooling it. The material is excellent inwear resistance and the like and also has high-strength and is excellentin heat resistance; however, the material does not apparently showpermeability that is an inherent characteristic of porous PTFE sheet,since the pores are filled with the resin.

Patent Document 11 discloses a sheet that is improved in both ofmoisture permeability and wear resistance. However, the sheet isproduced by a complicated process of forming a porous polyurethane mainlayer on a substrate, and then forming a surface layer on the mainlayer. In addition, it is difficult in such a production process tocontrol the condition for maintaining moisture permeability.Furthermore, the compressibility and heat resistance cannot be improvedin the sheet, since the pores of the porous layer are maintained as theyare.

PRIOR ART Patent Documents

-   Patent Document 1: JP 1-225652 A-   Patent Document 2: JP 4-214787 A-   Patent Document 3: JP 5-78645 A-   Patent Document 4: JP 2004-323717 A-   Patent Document 5: JP 2007-253519 A-   Patent Document 6: JP 2007-296756 A-   Patent Document 7: JP 2008-7607 A-   Patent Document 8: JP 2008-13654 A-   Patent Document 9: JP 2008-13715 A-   Patent Document 10: JP 2001-278997 A-   Patent Document 11: JP 2007-196184 A

DESCRIPTION OF THE INVENTION Problems to be Solved by the Invention

As described above, a variety of technologies for improving porous PTFEsheet have been developed so far. However, there has been no sheet thatexhibits both of mechanical strength and compressive resistance withoutimpairing advantages specific to PTFE including permeability, waterrepellency, heat resistance and chemical resistance.

It is an objective of the present invention to provide a method forproducing an air-permeable composite sheet which has excellentmechanical strength and compression resistance as well as PTFE's innateproperty such as air-permeability, water repellency, heat resistance andchemical resistance; and a filter and a fabrics material containing theair-permeable composite sheet produced by the method as a constituentmaterial.

Means For Solving the Problems

The present inventor made intensive studies to solve the above problems.As a result, the present inventor found that a composite sheet obtainedby filling pores of a porous PTFE sheet with a hardening materialsolution and then hardening or semi-hardening the sheet and furtherexpanding the sheet has a partially exposed PTFE and a continuous hole,and thus has inherent characteristics of the porous PTFE sheet such aspermeability and also exhibits improved mechanical strength, and furthercompressive resistance and abrasion resistance due to the hardeningmaterial, thereby completing the present invention.

The method for producing an air-permeable composite sheet according tothe present invention is characterized in comprising steps of fillingpores of a porous PTFE sheet with a hardenable material solution;hardening or semi-hardening the porous PTFE sheet filled with thehardenable material solution; and expanding the hardened orsemi-hardened porous PTFE sheet.

The filter according to the present invention is characterized incomprising the air-permeable composite sheet produced by the methodaccording to the present invention; and the fabrics material accordingto the present invention is characterized in comprising theair-permeable composite sheet. The filter and fabrics material hasproperties of the air-permeable composite sheet according to the presentinvention, such as excellent air-permeability, water repellency,mechanical strength, compression resistance and abrasion resistance.

MODE FOR CARRYING OUT THE INVENTION

The method for producing an air-permeable composite sheet according tothe present invention is characterized in comprising steps of fillingpores of a porous PTFE sheet with a hardenable material solution;hardening or semi-hardening the porous PTFE sheet filled with thehardenable material solution; and expanding the hardened orsemi-hardened porous PTFE sheet.

(1) Filling Step

In the present invention method, first, pores of a porous PTFE sheet arefilled with a hardenable material solution.

A porous PTFE sheet to be a raw material used in the present inventionis obtained by mixing a polytetrafluoroethylene fine powder with amolding aid in order to obtain a paste, obtaining a molded body from thepaste, expanding the molded body after removing or not removing themolding aid from the molded body, and optionally sintering the expandedbody. In the case of uniaxial expanding, the sheet has a fibrousstructure in which fibrils orient to the expanding direction and thereare pores between the fibrils. In the case of biaxial expanding, thesheet has an arachnoid fibrous structure in which fibrils radiallyextend and there are many pores surrounded by nodes and the fibrils.

The porous PTFE sheet is very useful as a material for a member whichhas to be produced in high temperature or used outside for a long time,since the PTFE sheet has properties such as heat resistance and weatherresistance.

The porosity of the porous PTFE sheet is not particularly limited solong as the sheet can be impregnated with a solution; and for securingimpregnation ability thereof, the porosity is preferably not less than30%, more preferably not less than 50%, and still more preferably notless than 70%. The porosity of the porous PTFE can be calculated withthe following equation, using the apparent density ρ (g/cm³) obtained bymeasurement in accordance with the method for measuring an apparentdensity defined in JIS K6885.Porosity(%)=[(2.2−ρ)/2.2]×100

The thickness of the porous PTFE sheet to be a raw material is notparticularly limited and may be properly determined depending on theintended use; however, the thickness is preferably not less than 1 μmand not more than 1000 μm. If the thickness is less than 1 μm, the sheetstrength may be insufficient and thus may be difficult to be handled;while if the thickness exceeds 1000 μm, the re-expanding after theimpregnation into a hardenable material solution may be difficult. Thethickness of the sheet is preferably not less than 10 μm and not morethan 500 μm, and more preferably not less than 20 μm and not more than200 μm. Moreover, the thickness is preferably adjusted to such an extentthat the sheet is capable of being wound in the form of a roll forallowing efficient production such as roll-to-roll processing. Ingeneral, a thin sheet may be called as a film, and a thin porous PTFEsheet may be handled as a porous PTFE film; however, in the presentinvention, a sheet is not particularly distinguished from a film, andthe term “sheet” is mainly used.

As the PTFE sheet to be a raw material, a PTFE sheet of whichfunctionality is improved by chemical modification or physicalmodification may be used. Chemical modification and physicalmodification methods are not particularly limited; and the chemicalmodification method is exemplified by a method of adding a functionalgroup to a fibril surface by acetylation, isocyanation, acetalization orthe like, and a method of covering a fibril surface with an organicsubstance or inorganic substance by chemical reaction. The physicalmodification method is exemplified by physical vapor deposition such asvacuum deposition, ion plating and sputtering, chemical vapordeposition, and plating methods such as electroless plating andelectrolytic plating. Only one of the modification methods may be used,or plural methods may be used in combination.

The porous PTFE sheet to be a raw material used in the present inventionmethod may have a single layer or a multiple layer. When a plural porousPTFE sheets to be a raw material are laminated, a hardenable materialalso plays a role as an adhesive to obtain a air-permeable compositesheet having a multiple layer.

The hardenable material used in the present invention is notparticularly limited so long as the material is hardened according tothe respective suitable condition. For example, hardenable sol-gelmaterials, thermoset materials and ultraviolet hardenable materials canbe used. Among them, hardenable sol-gel materials are suitable from theviewpoints of heat resistance and mechanical strength.

The term, “hardenable sol-gel material”, stands for a material thatincludes a soluble monomer or oligomer having relativelylow-molecular-weight, and is polymerized and hardened by polymerizationreaction. Such hardenable sol-gel materials can include metal alkoxidecompounds.

The metal element constituting metal alkoxide compounds may beexemplified by Si, Ti, Al, Sn, Zn and Mg. The alkoxy group constitutingmetal alkoxide compounds may be exemplified by C₁₋₆ alkoxy groups suchas methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, sec-butoxy andt-butoxy. As the metal alkoxide compound, a metal alkoxide compound ofwhich functionality is improved by chemical modification or physicalmodification may be used. The organic group for modification may beexemplified by C₁₋₂₀ alkyl groups and substituted C₁₋₂₀ alkyl groups;C₆₋₂₀ aryl groups and substituted C₆₋₂₀ aryl groups; C₇₋₂₀ aralkylgroups and substituted C₇₋₂₀ aralkyl groups; organic groups havingpolarity, such as —C—O—, —C═O, —COO—, —COOH, —CON═, —CN, —NH₂, —NH— andan epoxy group; and organic groups having an unsaturated carbon bond,such as >C═CH—.

As the metal alkoxide compound, silicon alkoxide compounds are suitable.The sheet of the present invention produced by impregnating with asilicon alkoxide compound solution and hardening the impregnated sheetis excellent in compressive resistance and mechanical strength. Inaddition, since a polymer of a silicon alkoxide compound is chemicallystable and excellent in heat resistance, weather resistance and thelike, such a polymer also can withstand the use in a hot process and inthe outdoor.

The silicon alkoxide compound may be exemplified bymethyltrimethoxysillane, methyltriethoxysillane, ethyltrimethoxysillane,ethyltriethoxysillane, propyltrimethoxysillane, propyltriethoxysillane,isobutyltrimethoxysillane, isobutyltriethoxysillane,diisobutyldimethoxysillane, dimethoxymethylsillane,phenyltriethoxysillane, methacryloxypropyltrimethoxysillane,aminopropyltriethoxysillane, aminoethylaminopropyltriethoxysllane,tetramethoxysillane, tetraethoxysillane, tetraisopropoxysillane,tetrabutoxysillane, and oligomers thereof. The silicon alkoxide compoundmay contain other metal alkoxide compound such as aluminium alkoxidecompound, titanium alkoxide compound and zirconium alkoxide compound, aslong as the silicon alkoxide compound contains silicon alkoxide as maincomponent, that is, not less than 50% of silicon alkoxide.

The term, “thermoset material”, stands for a material in whichpolymerization reaction is initiated by heating and which isirreversibly hardened by forming a three-dimensional cross linkedstructure between molecules. The thermoset materials is exemplified by,for example, thermosetting epoxy resins, phenol resins, melamine resins,urethane resins and unsaturated polyester resins.

The term, “ultraviolet hardenable material”, stands for a material inwhich polymerization reaction or crosslinking reaction is initiated byultraviolet irradiation to be hardened. The ultraviolet hardenablematerials is exemplified by, for example, epoxy acrylate, urethaneacrylate and polyester acrylate.

The ratio of a hardenable material in a solution may be properlyadjusted. For instance, the content amount of a final hardenablematerial in the sheet of the present invention depends on not only theporosity of the used porous PTFE sheet to be a raw material but also theconcentration of the hardenable material solution. Hence, theconcentration of the hardenable material solution can be adjustedaccording to the compressive resistance and the mechanical strengthdemanded for the sheet of the present invention. The concentration isgenerally preferably not less than about 20 wt % and not more than about95 wt %.

The solvent constituting the hardenable material solution used in thepresent invention method may be properly selected from solvents that candissolve the hardenable material to be used. For example, when a metalalkoxide compound is used as a hardenable sol-gel material, an alcoholis preferable. Alcohol is excellent in solubility for metal alkoxidecompounds and also can be readily distilled off after polymerizationreaction. In addition, for efficiently progressing sol-gel reaction, notless than about 0.2 mol/L and not more than 50 mol/L of water may beadded to the solution.

As the alcohol, methanol, ethanol, 1-propanol, 2-propanol and butanolare exemplified. Only one of them may be used or more than one may becombined for use. The mixing ratio may not be limited. The alcoholcorresponding to the alkoxide group in a metal alkoxide compound ispreferable. To the metal alkoxide solution, an acid or base may be addedas a catalyst for the polymerization reaction of a metal alkoxidecompound. As such an acid, hydrochloric acid, sulfuric acid, nitricacid, acetic acid and hydrofluoric acid are exemplified; and as such abase, sodium hydroxide, potassium hydroxide and ammonia are exemplified.

To the hardenable material solution used in the present inventionmethod, a metal oxide particle may further be added. The propertiesderived from the metal oxide particle to be added can be imparted to thecomposite sheet of the present invention. For example, the particle ofoxide of one or two or more metals selected from boron, aluminum,silicon, titanium, germanium, yttrium, zirconium, niobium, tantalum,zinc, indium, tin, barium, magnesium and lithium may be added; as aresult, the properties of the metal oxide particle can be imparted tothe present invention sheet. For example, linear coefficient of thermalexpansion and heat shrinkage factor of the sheet can be reduced.

The average particle diameter of the metal oxide particle is notparticularly limited; however, if the average diameter is too large, theparticle may be dropped out of the composite sheet. The average particlediameter is therefore preferably not more than 200 nm, more preferablynot more than 100 nm, more preferably not more than 50 nm, and stillmore preferably not more than 20 nm. The lower limit of the averageparticle diameter is not particularly limited; however, the averagediameter is, for example, not less than 1 nm. The narrower the particlesize distribution is, the better it is; and it is preferable that therebe no particle of which diameter exceeds 200 nm. The shape of the metaloxide particle is not particularly limited; however, the particle may besphere-shaped, rod-shaped, indeterminate-shaped or the like; theseparticles may be used alone or in combination of plural shapes. Two ormore kinds of different metal oxide particles may be blended together.

The blending amount of the metal oxide particle is not particularlylimited; and in general, the ratio is preferably not less than 10% bymass and not more than 90% by mass relative to the amount of thehardenable material solution. If the amount is less than 10% by mass,the effect of the added metal oxide particle may not be sufficientlyexerted. On the other hand, if the amount exceeds 90% by mass, there isa possibility that the polymerization reaction of the metal alkoxidecompound may not sufficiently proceed. The blending amount is morepreferably not less than 20% by mass and not more than 80% by mass, andstill more preferably not less than 30% by mass and not more than 60% bymass.

Additionally, to the hardenable material solution, can be added variousorganic-based or inorganic-based additives such as an ultravioletabsorber, an antimicrobial agent, an antistatic agent, a photocatalyst,a hardening catalyst other than the above, a plasticizer, a thickener, adefoamer, carbon black, and colorant such as a pigment and a dye.

The method for filling the porous PTFE sheet with the hardenablematerial solution is not particularly limited and common procedures canbe used. The method may be any one of, for example, vacuum pressureimpregnation, vacuum impregnation, spraying, evaporation to dryness, ametaling bar method, a die coating method, a gravure method, a reverseroll method and a doctor blade method. Even if the solution is appliedto the porous PTFE sheet only, the pores are filled with the solution.In other words, the term, “filling”, in the present invention is aconcept that the void of the porous PTFE sheet may be simply filled withthe solution and includes applying and the like as a means for thatpurpose.

When the porous PTFE sheet is thin, the pores of the porous PTFE sheetmay be filled with the solution only by the impregnation of one time. Onthe other hand, when the porous PTFE sheet is thick, the void may not beable to be completely filled with the solution only by the impregnationof one time. In such a case, the sheet is impregnated with the solutiona plurality of times so that the void is made to be completely filled.

The application method is not particularly limited, and may be, forexample, any method such as a metaling bar method, a die coating method,a gravure method, a reverse roll method and a doctor blade method.

When the solution is applied, it is preferable that the application iscarried out in a state in which the solvent of the surface to be appliedof the porous PTFE sheet is removed as much as possible. If theapplication is carried out in a state in which a solvent is attached tothe surface of the porous PTFE sheet, coating stains are likely tooccur, which may adversely affect the homogeneity and thickness of thesingle layer of the polymer of the metal alkoxide compound. In order toimprove adhesion of the porous PTFE sheet and the polymer layer, thesurface of the porous PTFE sheet can also be subjected to surfaceactivation treatment such as a corona treatment, a plasma treatment, aflame treatment and an alkali treatment.

(2) Hardening Step

Next, the porous PTFE sheet filled with the hardenable material solutionis hardened or semi-hardened.

Hardening or semi-hardening is carried out by hardening means inaccordance with the used hardenable material. For example, when ahardenable sol-gel material is used, a sol-gel reaction is conducted;and when a thermoset material or an ultraviolet hardenable material isused, heating or ultraviolet radiation is carried out, respectively. Theterm, “semi-hardening”, stands for not complete hardening; and thehardening extent may be adjusted by, for example, not sufficientlyremoving the solvent or heating in a sol-gel reaction, or by decreasingthe heating temperature or the amount of ultraviolet radiation or byshortening the heating time or ultraviolet radiation time in otherhardening means.

The sol-gel reaction is described in detail. For instance, when asilicon alkoxide compound is used, the sol-gel reaction is progressedthat generates a siloxane bond in oligomerizztion or polymerization withSi—O bond. The hardening method is not particularly limited as long asthe polymerization reaction proceeds; and the methods include a methodthat imparts energy to the material by irradiating ultraviolet light,x-rays, electron beams, infrared light or microwaves in addition to aheat treatment. It is preferable to carry out a heat treatment, sincethe treatment can be easily carried out.

The temperature of the heat treatment for hardening a hardenable sol-gelmaterial may be properly adjusted; and in general, the temperature isset at not less than about 20° C. and not more than about 320° C. Thehardening reaction may hardly proceed in some cases at less than 20° C.;on the other hand, when the temperature exceeds 320° C., crack may belikely to be generated and thus a good composite sheet may be hardlyobtained. The temperature is more preferably not less than 100° C. andnot more than 300° C., and still more preferably not less than 200° C.and not more than 300° C. The heating time may also be properlyadjusted, and generally the time is not less than about 10 minutes andnot more than about 360 minutes. When energy beam is radiated, the kindand strength thereof can also be properly selected.

When the polymerization reaction is carried out at a relatively lowtemperature, the material may further be heated preferably under reducedpressure to evaporate the remaining solvent.

(3) Expanding Step

Next, the hardened or semi-hardened porous PTFE sheet is expanded. Bythis step, the porous PTFE sheet consisting of nodes and fibrils ofwhich surface is intermittently covered with the hardenable material canbe obtained.

The expanding ratio may be properly adjusted, and is preferably not lessthan 1.1 times and not more than 20 times. If the expanding ratio isless than 1.1 times, the sheet may not become porous sufficiently, andthus there may be a fear that the permeability may not be kept. On theother hand, if the expanding ratio exceeds 20 times, disadvantages suchas fracture and necking may be generated and hence a good compositesheet may not be obtained. The expanding ratio is more preferably notless than 2 times and not more than 10 times.

As the expanding means, common procedures may be used; and for example,a method of passing the hardened or semi-hardened porous PTFE sheetbetween rolls having different rotating speed can be adopted. Theexpanding may be uniaxial expanding or biaxial expanding, both of whichmay be properly selected.

The step re-generates continuous holes that penetrate from the surfaceto the back in the porous PTFE sheet having pores filled with ahardenable material. The hardenable material is partially present on thesurface of such pores, and PTFE is exposed in the other parts. As aresult, the porous PTFE sheet maintains the inherent properties thereof,such as permeability and water repellency, and also shows improvedcompressive resistance. The presence or absence of the continuous holein the present invention sheet can be readily confirmed by an airpermeability tester or the like.

The content ratio of a hardenable material contained in the compositesheet of the present invention is preferably not less than about 10% bymass and not more than about 90% by mass. If the ratio of the hardenablematerial is less than 10% by mass, the compressive resistance and themechanical strength may not be sufficiently improved. On the other hand,if the ratio exceeds 90% by mass, the strength of the PTFE relative tothe hardenable material may be relatively decreased so that the sheetmay be fractured before sufficient expanding, and also the exposure ofthe PTFE may be excessively decreased so that the inherent properties ofPTFE, such as permeability and water repellence, may not be maintained.The ratio is more preferably not less than about 30% by mass and notmore than about 80% by mass.

(4) Re-Hardening Step

The expanded composite sheet may be re-hardened in order to furtherharden the hardenable material or harden the semi-hardened hardenablematerial. By such a re-hardening, further improvement of the heatresistance and mechanical strength of the composite sheet can beexpected so that the use under further high temperature or high pressureconditions may be possible. Such re-hardening can be carried out byheating.

The permeable composite sheet of the present invention produced by theabove method is improved in compressive resistance, mechanical strengthand abrasion resistance in addition to innate properties of the porousPTFE sheet including chemical resistance, heat resistance, permeabilityand water repellence. The PTFE sheet is therefore useful particularly asa filter material and a fabrics material.

The composite sheet of the present invention may be a single layer sheetor laminated sheet. The composite sheet of the present invention may bea laminated sheet produced by, for example, laminating a single layercomposite sheet according to the present invention to a single layercomposite sheet according to the present invention, oralternatingly-laminating another layer consisting of resin or aninorganic substance to a single layer composite sheet according to thepresent invention.

The number of layers and the thickness of each layers can be adjusted inaccordance with purposes. The above resin layer is preferably made from,for example, a fluorine resin, an acrylic resin, triacetyl cellulose orpolyvinyl alcohol. The inorganic layer is also not particularly limited;and the material thereof, for example, can be an oxide, nitride oroxynitride including one or more of Si, Al, In, Sn, Zn, Ti, Cu, Ce, Taand the like. The method for forming the resin layer and inorganic layermay be any method so long as an objective thin film can be formed; andthe example thereof includes physical vapor deposition such as asputtering method, a vacuum deposition method and an ion plating method,and an application method of forming a film by a chemical reaction usingheat energy and light energy, and chemical vapor deposition method.

When a conventional porous PTFE sheet is used as various filtermaterials, such a filter is excellent in chemical resistance and heatresistance; however, a filter with satisfied performance is neverobtained, since the pores thereof are broken or the filter itself isdamaged due to the pressure of the fluid or the collision of foreignmatter. However, the filter including the permeable composite sheetaccording to the present invention as a constituent material isexcellent also in compressive resistance and abrasion resistance.Therefore, although it is impossible for the conventional porous PTFEfilters to directly remove foreign particles or dusts adsorbed on thesurface with a brush, the filter according to the present invention isvery rarely damaged and the pores thereof are hardly collapsed, so thatthe above-mentioned maintenance is possible for the present inventionfilter.

When a conventional porous PTFE sheet is used as a fabrics material, forexample, used as rainwear, it is needed to make a special effort, sincethe porous PTFE is inferior in abrasion resistance. On the other hand,since the permeable composite sheet according to the present inventionis excellent in abrasion resistance and mechanical strength, the fabricsmaterial containing the present invention sheet is also excellent inabrasion resistance and mechanical strength in addition to permeabilityand water repellence. The term, “fabrics material”, in the presentinvention stands for a textile product such as fiber, fabrics and cloth.

The permeable composite sheet according to the present invention is alsoexcellent both in mechanical strength and compressive resistance inaddition to innate properties of PTFE including permeability, waterrepellence, heat resistance and chemical resistance. According to thepresent invention method, a permeable composite sheet having suchexcellent properties can simply and easily be produced. Therefore, thepresent invention relates to a sheet useful as a filter material, afabrics material or the like, and thus is industrially very useful.

EXAMPLES

Hereinafter, the present invention is described in detail with referenceto examples; however, it is not intended that the present invention belimited to the demonstrated examples, the present invention can bemodified in adherence with the spirit of the disclosure of thespecification in order to be carried out, and such modifications areincluded in the range of the present invention.

Example 1

A porous PTFE sheet (thickness: 60 μm, porosity: 70%) was cut out into25 cm×30 cm. The sheet was sufficiently impregnated with a silicasolution (silicon alkoxide solution manufactured by Nikko Inc., productname: Heatless Glass GS-600-1), and then taken out of the silicasolution. The solvent was distilled off by heating the sheet at 70° C.for 5 minutes until the silica solution became nonviscous. Next, thesheet was uniaxially expanded 4-fold. The sheet was further heated at100° C. for 3 hours for re-hardening in order to obtain a permeablecomposite sheet. When water was added dropwise to the obtained permeablecomposite sheet, water was repelled without infiltration into themembrane.

Example 2

A porous PTFE sheet (thickness: 165 μm, porosity: 70%) was cut out into25 cm×30 cm. The sheet was sufficiently impregnated with a silicasolution (silicon alkoxide solution manufactured by Nikko Inc., productname: Heatless Glass GS-600-1), and then taken out of the silicasolution. The solvent was distilled off by heating the sheet at 70° C.for 5 minutes until the silica solution became nonviscous. Next, thesheet was uniaxially expanded 4-fold. The sheet was further heated at100° C. for 3 hours for re-hardening in order to obtain a permeablecomposite sheet. When water was added dropwise to the obtained permeablecomposite sheet, water was repelled without infiltration into themembrane.

Example 3

A porous PTFE sheet (thickness: 100 μm, porosity: 70%) was cut out into25 cm×30 cm. The sheet was sufficiently impregnated with a silicasolution (silicon alkoxide solution manufactured by Nikko Inc., productname: Heatless Glass GS-600-1), and then taken out of the silicasolution. The solvent was distilled off by heating the sheet at 70° C.for 5 minutes until the silica solution became nonviscous. Next, thesheet was uniaxially expanded 1.5-fold. The sheet was further heated at100° C. for 3 hours for re-hardening in order to obtain a permeablecomposite sheet. When water was added dropwise to the obtained permeablecomposite sheet, water was repelled without infiltration into themembrane.

Test example 1 Air Permeability Test

The Gurley numbers of the permeable composite sheets produced in theabove Examples 1 and 2 were measured by using an Oken type airpermeability tester (manufactured by Asahi Seiko Co., Ltd., productname: KG1S) in accordance with the method of JIS P8117. In addition, theporous PTFE sheets used as raw materials in the above Examples 1 and 2were regarded as Comparative examples 1 and 2 respectively, and theGurley numbers thereof were similarly measured. The Gurley number meansthe time (sec) for 100 cm³ of air passes through a sample with an areaof 6.45 cm² at a pressure of 1.29 kPa in the vertical direction. Theresults are shown in Table 1.

TABLE 1 Comparative Comparative Example 1 example 1 Example 2 example 2Gurley number 1.2 2.8 9 17 (sec)

As shown in Table 1, although the permeable composite sheets accordingto the present invention included silica gel as a hardenable material,the sheets had sufficient air permeabilities due to being expanded, andpermeabilities thereof were rather higher than those of the porous PTFEsheets to be used as a raw material.

Test example 2 Compressive Resistance Test

The permeable composite sheets produced in the above Examples 1 and 3and the raw material porous PTFE sheets thereof (Comparative examples 1and 3) were compressed using a small press machine, and respectivecompressibilities were measured.

Specifically, first, the upper plate of the small press machine washeated to each temperature shown in Table 2, and each sheet waspressurized for 10 seconds at a press pressure of 40 kgf/cm² (about 3.9MPa). Additionally, the same procedure was carried out at ambienttemperature. After the press procedure, the decrease ratio of thickness(%) relative to the case where each sheet before pressurized was set at100 was calculated by the following equation as the criterion ofcompressive resistance. The results are shown in Table 2.The decrease ratio of thickness(%)=100−[(the thickness after the pressprocedure/the thickness before the press procedure)×100]

TABLE 2 temperature of press procedure ambient temperature 100° C. 200°C. 300° C. Comparative 35.5% 44.0% 53.7% 57.4% example 1 Example 1  1.0% 0.9%  0.0%  1.0% Comparative 10.3% 32.5% 32.2% 35.8% example 3 Example3  1.4%  2.7%  4.1%  5.4%

In addition, the decrease ratio of thickness (%) of each sheet when eachsheet was pressurized for 10 seconds at press pressures shown in Table 3without heating the upper plate was calculated as described above. Theresults are listed in Table 3.

TABLE 3 press presure 40 kgf/cm² 80 kgf/cm² 120 kgf/cm² 160 kgf/cm² (ca.3.9 MPa) (ca. 7.8 MPa) (ca. 11.8 MPa) (ca. 15.7 MPa) Comparative 21.6%44.3% 49.1% 51.8% example 1 Example 1  0.0%  1.0%  1.0%  1.9%Comparative 10.3% 40.2% 40.9% 45.7% example 3 Example 3  1.4%  4.0% 4.7%  4.0%

As in the above results, conventional porous PTFE sheets wereinsufficient in compressive resistance and were hardly returned to theformer state when pressurized. On the other hand, in the permeablecomposite sheet according to the present invention, the decrease ratioof thickness (%) was reduced to not more than about 5% or less even whenpressurized at high temperature and high pressure, indicating that thecompressive resistance is improved remarkably. Therefore, the permeablecomposite sheet according to the present invention is thought to be veryuseful as a filter material or a fabrics material that requirecompressive resistance.

Test Example 3 Mechanical Strength Test

The mechanical strengths of the permeable composite sheet produced inthe above Example 3 and the porous PTFE sheet (Comparative example 3) tobe a raw material were measured in the conditions of a sample width of 5mm and a speed of 50 mm/min by using a tensile testing machine(RTC-1210A, manufactured by ORIENTEC). The results are shown in Table 4.

TABLE 4 Comparative Example 3 example 3 Mechanical 14.5 6.5strength(N/mm²)

As the above results, it could be confirmed that the permeable compositesheet according to the present invention had a twice or more strength ascompared with the porous PTFE sheet to be a raw material.

What is claimed is:
 1. A method for producing one of an air-permeablefilter and an air-permeable fabrics material comprising, as aconstituent material, an air-permeable composite sheet, said methodcomprising: filling pores of a porous polytetrafluoroethylene (PTFE)sheet with a hardenable material solution comprising a hardenablematerial; hardening or semi-hardening the porous PTFE sheet filled withthe hardenable material solution to increase compression resistance andmechanical strength of the air-permeable composite sheet; and expandingthe hardened or semi-hardened porous PTFE sheet, after said hardening orsemi-hardening step, to increase porousness of the hardened orsemi-hardened porous PTFE sheet, wherein the expanding step re-generatescontinuous holes through the composite sheet, and wherein the hardenablematerial is present on the parts of the surface of the pores of the PTFEsheet and the PTFE sheet is exposed in other parts to provide waterrepellency on the exposed other parts.
 2. The production methodaccording to claim 1, further comprising: re-hardening the expandedporous PTFE sheet.
 3. The production method according to claim 1,wherein an expanding ratio is set to not less than 1.1 times and notmore than 20 times in the step of expanding the hardened orsemi-hardened porous PTFE sheet.
 4. The production method according toclaim 1, wherein a silicon alkoxide compound is used as the hardenablematerial.
 5. The production method according to claim 1, wherein thePTFE sheet has a porosity of not less than 30%, and a thickness of notless than 10 μm.
 6. The production method according to claim 1, whereinthe PTFE sheet has a porosity of not less than 70%, and a thickness ofnot less than 20 μm and not more than 200 μm.
 7. The production methodaccording to claim 1, further comprising: filling pores of at least oneadditional PTFE sheet with said hardenable material solution; andlaminating all of the porous PTFE sheets together.
 8. The productionmethod according to claim 1, wherein a concentration of the hardenablematerial in the hardenable material solution is not less than about 20wt. %.
 9. The production method according to claim 1, wherein thehardenable material solution further comprises: metal oxide particles.10. The production method according to claim 9, wherein the metal oxideparticles have an average particle diameter of not more than 200 μm. 11.The production method according to claim 9, wherein a ratio of theamount of the metal oxide particles to the hardenable material solutionis not less than 10% by mass.
 12. The production method according toclaim 1, wherein the hardening step comprises heat treatment of porousPTFE sheet filled with the hardenable material solution.
 13. Theproduction method according to claim 1, wherein a ratio of thehardenable material contained in the composite sheet is not less thanabout 10% by mass.
 14. A method for producing one of an air-permeablefilter and an air-permeable fabrics material comprising, as aconstituent material, an air-permeable composite sheet, said methodcomprising: filling pores of a porous polytetrafluoroethylene (PTFE)sheet with a hardenable material solution comprising a hardenablematerial; hardening or semi-hardening the porous PTFE sheet filled withthe hardenable material solution to increase compression resistance andmechanical strength of the air-permeable composite sheet; and expandingthe hardened or semi-hardened porous PTFE sheet, after said hardening orsemi-hardening step, to increase porousness of the hardened orsemi-hardened porous PTFE sheet, wherein the expanding step re-generatescontinuous holes through the composite sheet, wherein the hardenablematerial is present on the parts of the surface of the pores of the PTFEsheet and the PTFE sheet is exposed in other parts to provide waterrepellency on the exposed other parts, and wherein an expanding ratio isset to not less than 1.1 times and not more than 20 times in the step ofexpanding; and re-hardening the expanding porous PTFE sheet.
 15. Theproduction method according to claim 14 wherein the PTFE sheet has aporosity of not less than 70%, and a thickness of not less than 20 μmand not more than 200 μm.
 16. The production method according to claim15, wherein a concentration of the hardenable material in the hardenablematerial solution is not less than about 20 wt. %.
 17. The productionmethod according to claim 16, wherein the hardenable material solutionfurther comprises: metal oxide particles, wherein the metal oxideparticles have an average particle diameter of not more than 200 μm. 18.The production method according to claim 17, wherein a ratio of theamount of the metal oxide particles to the hardenable material solutionis not less than 10% by mass.
 19. The production method according toclaim 2, wherein the hardening step comprises heat treatment of porousPTFE sheet filled with the hardenable material solution, and wherein therehardening step comprises heat treatment of the expanded and hardenedor semi-hardened porous PTFE sheet.